Continuous digester with multiple column discharger



J. P. RICH CONTINUOUS DIGESTER WITH MULTIPLE COLUMN DISCHARGER Margh 25, 1969 Filed April 18. 1966 United States Patent 3,434,921 CONTINUOUS DIGESTER WITH MULTIPLE COLUMN DISCHARGER John Parker Rich, Nashua, N.H., assignor to Improved Machinery Inc., Nashua, N.H., a corporation of Delaware Filed Apr. 18, 1966, Ser. No. 543,095 Int. Cl. D21c 7/08 US. Cl. 162--237 4 Claims This invention relates to the continuous digestion of fibrous material and, more particularly, to novel apparatus for gradually reducing the pressure of the digested material as it is discharged from the pressurized digester.

The pulping of wood or other fibrous materials is carried out by digesting particles of fibrous material, such as wood chips, in a highly heated chemical solution, usually under pressure, as in the well-known sulphate process for the manufacture of wood pulp. In a continuous pulping process, the wood chips are continuously fed into the bottom of the reaction vessel or digester, treated by heating under pressure in a digester chemical solution as they move upward through the digester, and discharged at the top of the digester. The pressure on the pulp at the top of the digester, just before it is discharged therefrom, is often as great as 150 to 175 p.s.i.

Heretofore, the treated wood chips have generally been discharged from the digester through a special throttle or blow valve. Such valves create a very high velocity, corresponding to the pressure drop across the valve and dissipate much of the pulps internal energy in a very short period. It has become evident that the sudden pressure drop may injure the pulps strength or other desired characteristics. It is therefore a primary object of the present invention to provide novel discharge apparatus for gradually reducing the pressure and internal energy of the digested wood chips as they are discharged from the pressurized reaction vessel.

In order to accomplish this result, the invention provides a plurality of vertical standpipes, each of which is subjected to pulp upfiow, said standpipes being interconnected in series with the discharge of the reaction vessel by a plurality of downflow pipes. The wood chips, after being discharged from the reaction vessel and passing downwardly in free-fall through a series of connecting downflow pipes, move upwardly through each standpipe, falling from the top of one standpipe to the bottom of the next through the interconnecting downflow pipe. The pressure and internal energy of the pulp is thus decreased as it passes upwardly through each standpipe. The magnitude of the pressure drop is proportional to the height of the standpipe. In preferred embodiments, there is provided means for sensing the pulp level in each downflow pipe and reducing that level as required to insure pulp free-fall in that downflow pipe.

Other objects, features, and advantages will appear from the following description of a preferred embodiment of the invention taken together with the attached diagrammatic drawing thereof.

Referring more particularly to the drawing, there is shown conventional apparatus for digesting particles of wood or other fibrous material, generally designated 10, to which are connected novel means, generally designated 12, for gradually reducing the pressure of the digested pulp as it is discharged from digesting apparatus 10. As illustrated, digesting apparatus 10 comprises a relatively large, upright, pressurized reaction vessel or digester 16 having an inlet 18 adjacent the base thereof and a discharger 20 adjacent the top thereof. A hopper 22, in which the wood chips or other fibrous material may be stored, and a tank 24 for holding the digester chemical solution, are connected to inlet 18. A pump 26 is pro- 3,434,921 Patented Mar. 25, 1969 vided for forcing the wood chips and chemical solution into the base of reaction vessel 16.

The digested fibrous material or pulp is discharged from reaction vessel 16 into pressure reducing means 12 through discharge mouth 20. As illustrated, pressure reduction means 12 comprises a plurality of vertical, substantially cylindrical standpipes designated 28, 30 and 32, each of which has a pulp inlet adjacent the base thereof and pulp outlet adjacent the top thereof. The standpipes are interconnected in series with reaction vessel 16 by a plurality of substantially vertical downflow pipes designated 34, 36 and 38. The first downflow pipe 34 extends from discharge mouth 20 of reaction vessel 16 to the pulp inlet at the base of standpipe 28; the second downflow pipe 36 from the outlet at the top of standpipe 28 to the inlet at the base of standpipe 30; and the third downflow pipe 38 from the pulp outlet of standpipe 30 to the pulp inlet of standpipe 32. A pulp discharge pipe 40 is connected to the outlet at the top of the last standpipe 32.

To insure that each downflow pipe remains sufliciently empty to permit the digested pulp to fall freely from the top to the bottom thereof, pulp sensing and control means 42, 44 and 46 are provided in the lower position of downflow pipes 34, 36 and 38 respectively. These means sense the level of the pulp collected or backed-up in each downflow pipe and prevent the pulp level from exceeding a predetermined level by forcing the pulp from the downflow pipe, through the inlet of the next standpipe in the series.

In operation, the digested pulp is discharged from reaction vessel 16 through discharge mouth 20 into downflow pipe 34. After falling to the bottom of downflow pipe 34, it moves, successively, upwardly through standpipe 28, downwardly through downflow pipe 36, upwardly through standpipe 30, downwardly through downflow pipe 38, and upwardly through standpipe 32. The pressure and internal energy of the pulp is decreased as the pulp moves upwardly within each of the successive standpipes. The exact pressure drop within each standpipe depends on the height of the standpipe and weight of the pulp. The complete pressure reduction means 12 is preferably designed and constructed so that the pressure of the pulp at the top of standpipe 32, immediately before entering discharge pipe 40, has been reduced to substantially atmospheric level.

The weight of a typical digested pulp requires a standpipe from 2 to 3 feet tall to reduce the pulp pressure by 1 pound per square inch. Since the pulp pressure at the top of reaction vessel 16 may be as high as 175 p.s.i., the total required height of standpipes 28, 30 and 32 may be over 400 feet, or somewhat less than feet for each standpipe, about the height of a conventional reaction vessel with its accessory apparatus.

Other embodiments will occur to those skilled in the art and will be within the scope of the following claims.

What is claimed is:

1. In combination with a pressurized reaction vessel for producing pulp by chemically digesting particles of fibrous materials, means for gradually reducing the pressure of said digested pulp as said pulp is discharged from said vessel through a discharge mouth therein, said means comprising:

a plurality of substantially vertical standpipes, each of said standpipes including a pulp inlet adjacent the base thereof and a pulp outlet adjacent the top thereof; and,

a plurality of downflow pipes interconnecting said standpipes in series with said reaction vessel,

said digested pulp being discharged from said reaction vessel into the first of said downflow pipes at high pressure, moving successively downwardly through said downflow pipes and upwardly through said standpipes, and being discharged at substantially decreased pressure from the last of said standpipes in said series.

2. The apparatus of claim 1 including pulp sensing and control means for sensing the level of said pulp in each of said downfiow pipes and maintaining said pulp level below a predetermined level to insure substantially free-fall of said pulp within said downfiow pipes.

3. The apparatus of claim 1 including a discharge pipe connected to the pulp outlet of said last standpipe in said series.

4. In combination with a pressurized reaction vessel for producing pulp by chemically digesting particles of fibrous materials, means for gradually reducing the pressure of said digested pulp as said pulp is discharged from said vessel through a discharge means therein, said means comprising:

a plurality of substantially vertical standpipes, each of said standpipes including a pulp inlet adjacent the base thereof and a pulp outlet adjacent the top thereof;

a plurality of downfiow pipes interconnecting said standpipes in series with said reaction vessel, one of said downfiow pipes extending from said discharge means of said vessel to said pulp inlet of the first of said standpipes in said Series and one of said downfiow pipes extending from said pulp outlet of each of said standpipes other than the last of said standpipes in said series to said pulp inlet of the next of said standpipes in said series;

a discharge pipe connected to said outlet of said last standpipes in said series; and,

pulp sensing and control means for sensing the level of said pulp in each of said downfiow pipes and maintaining said pulp level below a predetermined level to insure substantially free-fall of said pulp within said downfiow pipes,

said pulp being discharged from said reaction vessel into said one downfiow pipe extending from said discharge mouth at high pressure, moving successively downwardly through said downfiow pipes and upwardly through said standpipes, and being discharged at substantially decreased pressure from said discharge pipe.

References Cited UNITED STATES PATENTS 2,707,146 4/1955 Burton l62l7 2,769,710 11/1956 Becker l62-17 3,035,962 5/1962 Schinn 162-17 HOWARD R. CAINE, Primary Examiner.

US. Cl. X.R. 

1. IN COMBINATION WITH A PRESSURIZED REACTION VESSEL FOR PRODUCING PULP BY CHEMICALLY DIGESTING PARTICLES OF FIBROUS MATERIALS, MEANS FOR GRADUALLY REDUCING THE PRESSURE OF SAID DIGESTED PULP AS SAID PULP IS DISCHARGED FROM SAID VESSEL THROUGH A DISCHARGE MOUTH THEREIN, SAID MEANS COMPRISING: A PLURALITY OF SUBSTANTIALLY VERTICAL STANDPIPES, EACH OF SAID STANDPIPES INCLUDING A PULP INLET ADJACENT THE BASE THEREOF AND A PULP OUTLET ADJACENT THE TOP THEREOF; AND, 